This invention relates to timing mechanisms, and more particularly to drive mechanisms for incrementally advancing the timing cam of an appliance timer.
Although the principles of the present invention may be included in various timers, a particular application is made in timers of the type employed to control the various circuits and functions of automatic laundry appliances, such as washers and dryers. Appliance timers basically comprise a rotatable member having a plurality of projecting cams which sequentially engage switch members, which, in turn, control the various operations of the appliance.
Basically, there are two types of timers for home appliances. One such timer is a cylindrical, rotary drum having a plurality of cams projecting from its surface which engage switch members, and is quite similar to a music box. An example of such an arrangement is shown in U.S. Pat. No. 3,395,585. Another such timer is described in U.S. Pat. No. 2,703,347. In the latter-mentioned patent, the timing cam comprises a flat disc mounted in a housing and having ratchet teeth on its periphery. The disc is driven incrementally about a central shaft by a pawl, which in turn is driven in a reciprocal manner by a synchronous motor. At least one face of the disc is provided with arcuate cam surfaces which are concentrically arranged about the face or faces of the disc. Switch members are arranged to engage the arcuate cam members upon rotation of the disc, and control various cycles of the appliance.
While the teachings of the present invention are set forth herein with particular reference to a disc cam, it is to be understood that the invention may also be employed as a drive mechanism for a drum cam. In either case, it is desirable that the disc or drum be driven at a very slow rate of advance, since one rotation of the disc or drum should complete at least one entire cycle of the machine.
A wide variety of mechanisms have been proposed for advancing the cam disc or drum in a step-by-step manner. Generally, all of these mechanisms include an array of ratchet teeth circularly arranged at the periphery of the disc or at the end of the drum which are engaged by a drive pawl and a "no-back" pawl which prevents reverse rotation of the ratchet teeth when the advancing pawl is retracted. The advancing pawl is incrementally driven by a cam or crank mechanism, and is retracted by a cam or spring mechanism while both pawls are entirely biased against the periphery of the ratchet by coil or cantilever wire springs. An example of a cantilever spring biasing the drive pawl into engagement with the ratchet may be found in U.S. Pat. No. 3,738,185 and an example of coil springs biasing the drive pawl into engagement with the ratchet may be found in U.S. Pat. No. 4,579,014. As may be appreciated from these patents, the assembly techniques involve precise hand operations and careful handling of the timer until the timer is fully assembled. Moreover, such springs tend to become displaced if the timer is jarred during installation or use.